Magnetics components are sometimes seen as a cure-all for noise problems, and they are also known for creating noise problems where none existed before. Many component choices are intended to target specific noise sources, but they don’t have to be taken in isolation. Most systems require multiple noise suppression mechanisms, often starting from the power stage and working downstream to the main loads in the system.

In particular, we want to look at two types of magnetic components that can create many problems or be helpful solutions in EMC design: inductors and transformers. The real behavior of these components can deviate significantly from the ideal behavior presented in electronics classes and textbooks. Accounting for real behavior in these components helps ensure a design will operate properly and with minimal noise problems.

Magnetics Selection Challenges for EMC

The major challenge in selecting appropriate components to ensure EMC is accounting for their parasitics. In power systems, both in power conversion and in the PDN for digital systems, parasitics are prominent and will produce all the undesired behavior seen during operation. The same idea applies to filters that may be applied in these systems, such as the post-regulation filter or an output low-pass filter on a power supply.

Transformers

Transformers and inductors share many of the same problems in terms of parasitics and undesirable magnetic behavior. However, transformers do have some unique issues that can compromise safety. In addition to winding capacitance that allows coupling into a single winding, there is interwinding capacitance that allows noise to conduct across windings. The various capacitances are shown below.

 

 

There are two solutions to the winding problem in a transformer: use larger spacing between windings and separate the windings by a larger distance. This would also increase the isolation across the gap between the primary and secondary sides, which is beneficial for ESD protection in real high voltage systems. By increasing the gaps, you decrease the capacitance and increase the impedance to high frequency noise traversing the gap across the transformer.

Unfortunately, increasing the gaps within and between windings decreases system efficiency. In a transformer-coupled switching DC-DC converter, such as a flyback converter, or in an LLC resonant converter in an isolated power supply, the inductive coupling strength is a major determinant of power conversion efficiency. Separating these elements requires a physically larger core, which makes the system heavy and bulky. It also decreases the field strength and creates additional leakage inductance that allows the magnetic field to couple signals into nearby circuits. Both are detrimental to noise and power conversion efficiency.

It is the author’s opinion that transformers should be chosen with an eye towards power conversion efficiency and safety first, followed by issues like preventing noise coupling. The best solutions include:

• Use an isolated topology, even if the design does not need one for safety concerns, and provide a feedback loop using an optoisolator.
• Place a safety capacitor in parallel with the transformer, but make sure the safety capacitance is much larger than the transformer’s total equivalent capacitance.
• Apply ESD protection circuits to any IOs with exposed conductive connectors.
• Layout the board such that return currents are properly tracked around the layout.

With these steps, you may find that physically larger transformers are not needed unless power efficiency or thermal handling are the primary concern. When operating at much higher frequencies involved in RF power supplies, layout and placement will be the major drivers of noise suppression.

Inductors and Ferrite Beads

I lump ferrite beads and inductors together as they essentially perform the same functions once you account for the inductor’s parasitics. Inductors essentially behave as parallel RLC circuits. They have some DC resistance in the wiring that limits their current, as well as parallel winding capacitance that acts as a high pass element. In total, the equivalent behavior of a typical inductor is as a bandpass circuit, just like a ferrite.

 

 

For a ferrite bead, the inductor model could be significantly modified to account for the high DC resistance at the component’s resonant frequency.

From the above model, we can see that an inductor and ferrite will provide capacitive impedance at sufficiently high frequencies, and this needs to be accounted for at high frequencies if the components are to be used in filters. In a voltage regulator that is intended to supply power to low frequency loads, or possibly to DC loads, the capacitive behavior essentially doesn’t matter; the relevant frequencies will be low enough that the component appears inductive and will prevent noise from reaching the load. Coupled with a shunt capacitor, and the current reaching the load will be decidedly DC.

In the case of a digital system, ferrites and inductors are undesirable except in cases of isolating two rails on the same component. Even in this case, the design should be tested as I have seen mixed results regarding the use of ferrites to isolate PLL rails from core voltage rails. In my experience, designs involving large high-speed digital ICs can still work fine without any ferrites as long as the PCB stackup is constructed correctly, meaning there is sufficient capacitance throughout the entire signal bandwidth (reaching well into GHz frequencies).

Regardless of the use of ferrite beads and inductors in filters, they are still necessary in power conversion and should be selected appropriately. The following specifications are critical:

• Current limit
• Saturation current
• Temperature limits
• Winding capacitance
• Leakage inductance

The 1st and 2nd points are quite important but they are not necessarily the same thing. The saturation current defines the current at which a ferrite bead or the ferrite core in an inductor will saturate and exhibit a nonlinear response, followed by hysteresis as the core demagnetizes.

Ringing

When measuring the current on a single switching node, one will find that magnetic components exhibit ringing, or an underdamped oscillation. This is particularly noticeable if the system happens to enter the discontinuous conduction mode, where the current will fall to zero and exhibit a strong transient response. This occurs because current-controlled switching action is driving the transient response in an equivalent RLC circuit. To eliminate this problem, some small amount of resistance is needed to dampen the transient response and, ideally, place it into the critically damped regime.

 

 

I’ve only focused on magnetics in the previous paragraph, but the same ideas regarding transient response apply to capacitors. All capacitors behave like RLC circuits thanks to their equivalent series inductance (ESL) and equivalent series resistance (ESR). In the case where there is a large voltage-controlled switching action and transient response, there will be a large current fluctuation measured at the node connecting to the capacitor. Some damping is also needed here, sometimes being as small as a few Ohms. Controlled ESR capacitors are also useful here as they have higher-than-normal ESR values and have a reliable specification.

Conclusion

The component choices shown above address multiple noise sources that may be present in EMC design challenges as they relate to magnetics components. The important takeaway from the above guide is to choose transformers and inductors with the appropriate coil inductance values, either by considering coil turns, physical form factor or size, or core ferrite material. Parasitics and topology also play a role in selection of inductors and transformers, as well as the physical layout on a PCB.

Even if noise is relatively severe, proper PCB layout can provide some suppression against noise and will help ensure a system operates as intended. We’ll show more on this in an upcoming article.

 

Whether you're designing high-speed PCBs for mil-aero embedded systems or a complex RF product, you should work with a design and development firm that can ensure your product will be reliable and manufacturable at scale. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your next high speed digital system is fully manufacturable at scale. Contact NWES for a consultation.

 

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